In a conventional rolling stand, for example in a four-high or six-high stand with both positive and negative bending of the working rolls WR, the bending force is generally managed, for each chock, by means of four, six or eight actuators or hydraulic jacks A (FIG. 1a).
The actuators A are single effect and therefore two (or four) of them thrust the upper chock GWR upwards (positive bending), while two (or four) of them thrust the same upper chock GWR downwards (negative bending).
Normally, considering the forces in play required to perform an effective “mechanical” rounding of the working rolls, the piston of each actuator is rather bulky, so that this solution is limited with regard to the maximum bending force applicable to the working roll. Moreover, this conventional solution requires that seatings be made (in the housing or in blocks connected to the housing) above and below the fins D of the chocks GWR of the working rolls WR. The bulk may be such that the solution with both positive and negative bending becomes difficult, especially in stands where the working rolls WR must also be displaced axially (shifting).
Very often, considering the limits of space, it is necessary to manage without applying negative bending, thus considerably reducing (by about a half) the capacity to dynamically control the strip profile which can be obtained with both positive and negative bending.
The presence of play and friction usually causes an indetermination in the passage from negative bending to positive bending and vice versa, unless particular strategies are adopted such as the one we shall now illustrate.
With reference to the diagram in FIG. 2, which refers to a conventional embodiment, for example that shown in FIG. 1a, where the x-axis shows the desired total bending force (FTD) and the y-axis shows the actual total bending force (FTE), if the total bending has to be negative, pressure is applied to the negative bending hydraulic jacks and the positive bending hydraulic jacks are kept on zero. If the total bending is to be positive, a symmetrically opposite approach is used.
As we have seen, however, due to the play and friction, a management of this type creates an area of indetermination AI, or grey area, around the zero.
In other words, it is impossible to be sure of the real value of the actual total bending force (FTE) in the shaded area AI in FIG. 2, with respect to the desired bending force (FTD).
In a conventional embodiment, for example that shown in FIG. 1a, to eliminate the problem illustrated above and to obtain a substantially linear resulting force F (FIG. 3), it is possible to manage the forces as follows.
When the total bending is positive, the negative bending hydraulic jacks generate a constant negative force, while simmetrically the opposite happens when the total bending is negative.
The state of the art comprises document JP-A-62009708, which discloses a roll bending device wherein, for each roll chock, a plurality of double-effect actuators are interposed between an upper fin and a lower fin of the chock. When the working rolls are rearranged, a lower reinforcing roll is moved down and an upper reinforcing roll is moved up by a balance cylinder. By means of two actuators, the chock of the upper working roll is moved up and the chock of the lower working roll is moved down to the lowermost position. By that operation, a first lateral wheel of the chock of the lower working roll is ridden on a rearranging rail, so that the same chock is able to be pulled out. On the other hand, the upper fin of the chock of the upper roll is supported by the piston rod of the corresponding actuator and a first gap is generated between the arm and the piston rod of the other actuator. A second gap is generated between a second wheel set on the upper fin of the chock of the upper working roll and a rearranging rail. This device, even if it uses double-effect actuators, is very complicated and cumbersome and does not provide for the control of small forces to obtain bending around the zero position.
The present Applicant has devised the device according to the invention to overcome these shortcomings.